The present invention relates to the manufacture of different cooking liquors (or like liquids) utilizable in a cellulose pulp mill by green liquor crystallization. The method is based upon the simple but effective and energy-efficient separation of sodium carbonate from green liquor.
In sulphate cooking wood is traditionally treated with "white liquor" containing sodium hydroxide and sodium sulphide. Lignin is dissolved and cellulose fibers are released. A mixture of cellulose fibers (pulp) and cooking chemicals is treated with water, producing "black liquor". Black liquor containing dissolved lignin and cooking chemicals is evaporated and then combusted in a recovery boiler (or gasified, or otherwise treated), to recover energy and chemicals. Depending on the combustion technique used, chemicals are obtained in a molten or solid phase, which are dissolved so as to form "green liquor" containing sodium sulphide and sodium carbonate. Usually the green liquor is causticized with caustic lime (CaO) to white liquor containing sodium hydroxide and sodium sulphide.
Typically, the sulphidity of cooking liquor has been 30-40% (sulphidity refers to the ratio of the amount of sodium sulphide to the total amount of sodium sulphide and sodium hydroxide). It is, however, known that by increasing the sulphidity of the white liquor, it is possible to produce pulp having a higher viscosity and better physical properties. It is also known that the sooner in the cooking sequence the sulphidity of white liquor becomes high, the clearer the advantages. Consequently, it is desirable to provide cooking liquors with different sulphidities at different stages of cooking. White liquor with high sulphidity (i.e. at least about 40%) is used at the beginning of cooking, and liquor with low sulphidity (e.g. less than about 30%) or normal sulphidity is supplied to the later stages of cooking. In the most extreme cases white liquor may contain only sodium hydroxide as an active substance.
Several technical and practical features have prevented the utilization of different sulphidity white liquors in commercial chemical cellulose pulp manufacture. One problem has been how to manufacture different sulphidity white liquors in a highly energy-efficient manner.
It is possible to use green liquor to produce green or white liquor with high sulphidity and liquid containing substantially only sodium hydroxide by crystallizing sodium carbonate out from green liquor, and causticizing it. The sodium hydroxide solution made from the separated sodium carbonate crystals may be used in different parts of the pulp mill.
The crystallization of sodium carbonate in the green liquor may be practiced by conventional evaporation techniques, such as by rising heat for evaporating water and raising thus the sodium carbonate content in the green liquor above the crystallization level. However if this kind of evaporation is employed the investment and operation costs must be optimized. If, for example, a multiple effect evaporation plant is used, the number of the effects decreases the amount of the primary steam required but adds significantly to the apparatus/equipment costs (initial investment and maintenance).
Conventional evaporation is suitable when the object is to increase concentration, for example, to improve the combustibility (black liquor). The purpose in the green liquor crystallization is, however, to separate sodium carbonate. It is known that the solubility of sodium carbonate in green liquor is at the lowest level at low temperature (&lt;20.degree. C.), at which solubility is less than 10 weight-%. Thus the crystallization of sodium carbonate is easier at these low temperatures. If crystallization were carried out directly as vacuum evaporation, the problem would be a low temperature level. For the energy economics of a pulp mill, it is desirable that the temperature of the final products be as high as possible.
A purpose of the present invention is to provide a method for the manufacture of cooking liquors of at least two different sulphidities in an as energy-efficient manner as possible.
According to one aspect of the present invention a method of manufacturing cooking liquor for digesting comminuted cellulosic material to produce chemical cellulose pulp, black liquor being produced during the production of chemical cellulose pulp, is provided. The method comprises the following steps: (a) Treating black liquor to recover chemicals therefrom. (b) Dissolving the chemicals from step (a) to produce green liquor. (c) Decreasing the temperature of the green liquor from step (b) to effect crystallization of sodium carbonate in the green liquor by expanding the green liquor in at least two stages, vapor being produced during expansion. (d) Separating the sodium carbonate crystals produced during the practice of step (c) to produce a green liquor with high sulphidity. (e) Heating the high sulphidity green liquor from step (d) with at least part the expansion vapor produced during step (c) by bringing the expansion vapor and high sulphidity green liquor into heat exchange relationship. And, (f) dissolving the sodium carbonate crystals separated in step (d) to produce a low sulphide content alkaline solution.
Step (e) is preferably practiced by bringing the vapor and green liquor into direct heat exchange relationship. Step (e) may be practiced using a first part of the expansion vapor from step (c), in which case there is a further step (g) of heating the low sulphide content alkaline solution from step (fi by bringing it into indirect heat exchange relationship with a second part of the expansion vapor from step (c). Step (c) is preferably practiced by expanding the green liquor in at least three expansion stages including a last, lowest temperature, expansion stage, and there are the further steps of (h) recovering heat from the last expansion stage (for example by using a heat pump system), and (i) using the heat recovered in step (h) to assist in the practice of step (fl.
In a typical method according to the present invention, step (a) is practiced by burning black liquor in a recovery boiler, or by gasifying black liquor, although other known techniques may be used. There is also typically the further step of clarifying the green liquor between steps (b) and (c), and step (c) is typically practiced using more than three stages. Also the high sulphidity green liquor and the low sulphide content alkaline solution may either or both be causticized.
There is also typically the further step (j) of dividing the green liquor from step (b) into first and second portions, the first portion used in the practice of steps (c)-(e), and the second portion treated to produce a second stream of green liquor having a sulphidity at least 10% (and preferably at least about 20%) lower than the high sulphidity green liquor from step (e). Steps (a) through (e) may be practiced to produce as the high sulphidity green liquor a green liquor having a sulphidity of about 60-70%, and the second stream of green liquor will typically have a sulphidity of about 30-40%, although it can be made with lower sulphidity (e.g. 20% or less).
Step (c) may be practiced to reduce the temperature of the green liquor from about 85-95 (e.g. about 90.degree. C.) to about 14-20 (e.g. about 16.degree. C.) between the first and last stages. Step (e) may be practiced to produce high sulfidity green liquor having a temperature of about 65-75 (e.g. about 69.degree. C.), while step (g) is practiced to heat the low sulphide content alkaline solution to a temperature of about 65-75 (e.g. about 69.degree. C.) Step (c) is typically practiced using flash tanks for at least some of the stages, and typically all of them.
According to another aspect of the present invention a method of manufacturing cooking liquor is provided comprising the following steps: (a) Treating black liquor to recover chemicals therefrom. (b) Dissolving the chemicals from step (a) to produce green liquor, and clarifying the green liquor so produced. (c) Dividing the clarified green liquor from step (b) into first and second portions. (d) Decreasing the temperature of the first portion of the green liquor to effect crystallization of sodium carbonate in the green liquor by expanding the green liquor in at least two stages, vapor being produced during expansion. (e) Separating the sodium carbonate crystals produced during the practice of step (d) to produce a green liquor with high sulphidity. (f) Dissolving the sodium carbonate crystals separated in step (e) to produce a low sulphide content alkaline solution. And, (g) using the second portion of green liquor from step (c) to produce a second stream of green liquor having a sulphidity at least 10% lower than the high sulphidity green liquor from step (e). This method also typically comprises the further steps of: bringing a first part of the expansion vapor from step (d) into direct heat exchange relationship with the high sulphidity green liquor from step (e) to heat the high sulphidity green liquor; and heating the low sulphide content alkaline solution from step (f) by bringing it into indirect heat exchange relationship with a second part of the expansion vapor from step (d).
It is the primary object of the present invention to provide an effective energy-efficient method for producing high sulphidity cooking liquor. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.